The present invention relates to a process for catalytic cracking of hydrocarbons, in which two stages of catalyst regeneration are employed. In one aspect, the present invention concerns a process for catalytic cracking of heavy hydrocarbon stocks, such as petroleum residuals, and regenerating spent catalyst containing a substantial coke concentration.
Conventional hydrocarbon cracking operations employ moving beds or fluidized beds of catalyst. Catalytic cracking is carried out in the absence of externally supplied molecular hydrogen and without substantial hydrogen consumption, as distinguished from hydrocracking. In catalytic cracking, an inventory of particulate catalyst is continuously cycled between a cracking reaction step and a catalyst regeneration step. In conventional fluidized catalytic cracking (FCC) systems, a stream of fluidized catalyst particles is contacted with a hydrocarbon feed in a cracking zone or reactor at a temperature of about 425-600.degree. C., usually about 460-560.degree. C. Cracking of hydrocarbons at the elevated operating temperature results in deposition of carbonaceous coke on the catalyst particles. The fluid products resulting from the cracking step are separated from the coke-deactivated, spent catalyst particles, and the products are recovered. Spent catalyst is stripped of volatiles, usually with steam, and passed to a catalyst regenerator. To regenerate the catalyst, it is conventionally contacted with a predetermined amount of molecular oxygen in a fluidized bed. A desired portion of the coke is burned off the catalyst, restoring the activity of the catalyst and heating it to an elevated temperature, e.g., 540- 815.degree. C., usually 590-730.degree. C. Flue gas formed by combustion of coke during regeneration may be treated for removal of particulates and conversion of carbon monoxide to carbon dioxide, after which the flue gas is normally discharged into the atmosphere.
Commercial FCC feeds are typically gas-oils boiling in the range from about 221-400.degree. C. In a few cases, heavier feeds are used, usually mixed with at least some gas-oil. Heavier fractions, such as atmospheric and vacuum residual stocks, are relatively abundant and inexpensive. When the supply of gas-oil FCC feeds is low, use of heavy residual fractions for FCC conversion is particularly advantageous. The term "residual petroleum fraction", as used herein, may be defined as a fraction of petroleum boiling above about 400.degree. C., although the overhead from vacuum distillation may have a boiling point up to about 570.degree. C.
Residual petroleum fractions usually contain metals such as nickel, vanadium and iron. Residual fractions also typically contain heat-sensitive and non-distillable components, such as asphaltenes, which tend to form coke and hydrogen when heated. The presence of metals and asphaltenes makes residual fractions relatively difficult to process in conventional FCC systems. Metals tend to accumulate on the cracking catalyst and adversely affect the selectivity of the catalyst, with a resulting increase in coke and hydrogen yield. Asphaltenes likewise tend to form coke and hydrogen upon breakdown during the cracking operation. Large coke and hydrogen yeidls are quite undesirable. The selectivity of feed conversion to desired products is an important economic parameter in FCC operations. The primary desired product is a naphtha fraction, approximately a 24-220.degree. C. cut, while coke and hydrogen are primarily undesired by-products. Thus, processing of residual feeds presents a problem of a cumulative effect on the amount of coke generated on the catalyst due to the metals and asphaltenes.
Formation of a certain amount of coke during the cracking step is necessary to supply process heat. On the other hand, when the amount of coke necessary to supply heat has been burned off the catalyst, the concentration of coke on the regenerated catalyst must be low enough so that the catalyst is active. When the amount of coke on the spent catalyst substantially exceeds the amount of coke needed to maintain the cracking unit in heat balance, it is difficult, in conventional processing systems, to burn off all the excess coke and dispose of the resulting heat energy. Riser-type (entrained catalyst bed) cracking has been suggested for processing residual feeds, to shorten the catalyst-oil contact time and thereby reduce coke make. Usually, riser cracking only partially solves the problem of excessive coke formation. Prior hydrogen processing of cracking feeds to convert heat-sensitive, coke-forming components of the feed before catalytic cracking has also been suggested, both for demetallation and for converting asphaltenes. The benefits of prior hydrogen treatment are often at least partially offset by the attendant expense.
In regenerating heavily coked spent catalyst, it can be difficult to burn off enough coke to provide a suitably low concentration of carbon on regenerated catalyst. In general, a relatively low concentration of carbon on regenerated catalyst is advantageous to a cracking operation, since a low level of carbon on regenerated catalyst normally provides a more selective, active catalyst, particularly for catalysts containing a zeolite cracking component. Conventional cracking catalyst regeneration is typically carried out in a single-stage operation, using a dense-phase fluidized bed of catalyst particles. Some conventional, single-stage regeneration systems regenerate catalysts in an incomplete carbon monoxide combustion mode, which usually leaves a relatively high concentration of carbon on the regenerated catalyst generally more than 0.2 weight percent, and often about 0.25 to about 0.45 weight percent. Flue gas removed from cracking catalyst regenerators operating in an incomplete combustion mode is characterized by relatively low carbon dioxide/carbon monoxide volume ratio. The amount of oxygen introduced into a catalyst regenerator operating in an incomplete combustion mode must usually be carefully limited in order to prevent afterburning, combustion of carbon monoxide in the flue gas downstream of the dense bed of catalyst in the regenerator, which can result in overheating of the flue gas. Typically, the carbon monoxide is burned in a boiler to dispose of it before discharging the flue gas. Other conventional cracking systems regenerate catalyst in a complete carbon monoxide combustion mode, in which carbon monoxide and coke are essentially completely burned in a single stage of regeneration, carbon monoxide, combustion being promoted either thermally or by an active combustion-promoting metal circulated with the cracking catalyst.
In a few cases, plural stages of catalyst regeneration have been suggested in the cracking art. For example, U.S. Pat. No. 3,494,858 describes partial regeneration of catalyst in a first fluidized bed, using flue gas produced by a second-stage, transfer line regeneration. U.S. Pat. No. 3,767,566 also shows plural regeneration stages, with a first regeneration stage using a dilute fluidized catalyst phase and flue gas from a second regeneration stage, the second stage using a dense fluidized catalyst phase. Use of riser-type regenerators has been suggested in U.S. Pat. No. 2,929,774, in connection with a countercurrent contact cracking reactor.